Upgrade Symfony 3.x to 4.4.x (GoogleCloudPlatform#1011)

- [x]  Remove deprecated code for Symfony >= 4.3
- [x]  Upgrade SF framework from 3.x to 4.4.x.

Deprecated changes:
* The class `GetResponseForExceptionEvent` is deprecated since Symfony 4.3, use ExceptionEvent instead.
* Method `$event->getException()` is deprecated since Symfony 4.4, use getThrowable instead

Author: comxd

appengine/flexible/symfony/README.md

@@ -17,7 +17,7 @@ Before setting up Symfony on App Engine, you will need to complete the following
 Use composer to download Symfony Standard and its dependencies
 
 ```sh
-composer create-project symfony/framework-standard-edition:^3.0
+composer create-project symfony/framework-standard-edition:^4.4
 ```
 
 # Integrate Stackdriver

appengine/flexible/symfony/src/AppBundle/EventSubscriber/ExceptionSubscriber.php

@@ -6,7 +6,7 @@
 
 use Google\Cloud\ErrorReporting\Bootstrap;
 use Symfony\Component\EventDispatcher\EventSubscriberInterface;
-use Symfony\Component\HttpKernel\Event\GetResponseForExceptionEvent;
+use Symfony\Component\HttpKernel\Event\ExceptionEvent;
 use Symfony\Component\HttpKernel\KernelEvents;
 
 class ExceptionSubscriber implements EventSubscriberInterface
@@ -19,9 +19,9 @@ public static function getSubscribedEvents()
         ]];
     }
 
-    public function logException(GetResponseForExceptionEvent $event)
+    public function logException(ExceptionEvent $event)
     {
-        $exception = $event->getException();
+        $exception = $event->getThrowable();
         Bootstrap::exceptionHandler($exception);
     }
 }

appengine/flexible/symfony/test/DeployTest.php

@@ -69,7 +69,7 @@ private static function verifyEnvironmentVariables()
     private static function createSymfonyProject($targetDir)
     {
         // install
-        $symfonyVersion = 'symfony/framework-standard-edition:^3.0';
+        $symfonyVersion = 'symfony/framework-standard-edition:^4.4';
         $cmd = sprintf('composer create-project --no-scripts %s %s', $symfonyVersion, $targetDir);
         $process = self::createProcess($cmd);
         $process->setTimeout(300); // 5 minutes